Nanoparticle suspensions from carbon-rich fluid make high-grade gold deposits

Economic gold deposits result from a 100- to 10,000-fold enrichment in gold relative to crustal background. In hydrothermal systems, this enrichment is achieved through the transport and accumulation of metals via deeply sourced fluids to a site of deposition. However, the generally low metal solubility of Au in aqueous solutions in orogenic systems requires additional processes in order to explain high-grade gold formation. Reports of Au nanoparticles in high-grade gold veins infer that their formation is linked to mineralisation. However, processes leading to nanoparticle nucleation and deposition remain poorly understood. Here we show that formation of metal nanoparticles (Au, AuAg, Cu, Ag2O) is one of the essential contributors to efficient and focused gold deposition. We report systematic and previously unrecognized metal nanoparticles preserved in amorphous silica and/or carbonic phases in five high-grade deposits. The association of metal, silica and carbonic phases helps to constrain the multiple reactive processes involved in Au, Cu and Ag metallogenesis and formation of high-grade gold mineralisation.


Descriptions of Deposits and Samples
A summary of the characteristics of each deposit part of this study is presented in Table 1 and further details on each deposit is presented hereafter.

Deposit details
Red Lake is a producing gold mine that is part of the Mesoarchean Red Lake greenstone belt located in the Superior Province, Western Ontario, Canada. Campbell-Red Lake is a world-class deposit with a total production of approximately 840 tons of gold and characterized by exceptionally high-grade mineralisation with previous average gold grades up to 88 g/t Au 1,2 . The high-grade mineralisation is mainly hosted in tholeiitic basalt locally intercalated with komatiitic basalt 3 which have been metamorphosed to middle greenschist and amphibolite facies 4 . The mineralisation occurs in iron-carbonate -quartz veins and breccias that host native gold, fine arsenopyrite, disseminated pyrite, pyrrhotite, magnetite and traces of fine-grained chalcopyrite and sphalerite 2,3 . Gold mineralisation occurs within quartz veins estimated to have been emplaced at temperatures that range from 350° to 550°C and pressures up to 257 Mpa based on fluid inclusion studies on quartz related to mineralisation 1,5 , corresponding to a depth close to 9 km deep. An accurate estimate of the mineralisation emplacement depth is precluded by uncertainty on fluid temperature but 5 study suggests a depth of mineralisation emplacement > 5km deep. The high-grade mineralisation is interpreted to have formed between ca. 2723 and 2712 Ma based on U-Pb geochronologic data combined with detailed mapping and crosscutting relationships 2 .

Sample description
The sample selected for this study is a gold-rich quartz vein from the Red Lake mine ( Figure 1A). The three main components of the sample are, in order of abundance: quartz, visible gold and arsenopyrite. The quartz is crystalline and fine-grained, and the gold is coarse-grained and associated with fine-grained arsenopyrite.
Electron microscopy backscatter images show that gold is either associated with euhedral arsenopyrite (between 5 µm to 0.5 mm) or, to a lesser extent, pyrite and pyrrhotite ( Figure 1B, C). Gold occurs as free coarse grains in the quartz veins but also forms rims around arsenopyrite and is found in micro-fractures in arsenopyrite. Berthierite (FeSb2S4) is associated with gold and arsenopyrite. Inclusions, present in the gold grains, contain silica and/or carbon. These inclusions appear sub-rounded at the surface of the sample, represent < 1 vol. % of the quartz grains and are under 3 µm in diameter ( Figure 1D).

Deposit details
Beta Hunt is a producing nickel and gold mine that is located in the Kambalda-St Ives region in Western Australia within the central portion of the Norseman-Wiluna greenstone belt 6 . The Beta Hunt mine is atypical for hosting two genetically-unrelated types of mineralisation in different parts of the mine: komatiite-related massive sulfides and orogenic-style gold locally preserving exceptional high-grade intercepts containing up to 20% Au 7 . At Beta Hunt, gold mineralisation is found in quartz veins intersecting meta-basalt intercalated with graphitic metasedimentary rocks metamorphose at greenschist facies 8,9 . The gold mineralisation is believed to have been emplaced at mid-crustal conditions; temperature and pressure were estimated from fluid inclusion studies and returned temperatures that range from 285 to 350°C and fluid pressures that range from 80 to 140 MPa, corresponding to a depth of formation greater or equal to 3 km deep 9,10 . No direct dating exists for the Beta Hunt deposit, however, age of mineralisation is estimated to have occurred between ~2675 Ma and ~2640Ma based on nearby dated gold mineralisation 11,12 .

Sample description
The sample studied (SLM191122-001) is a gold-rich quartz vein from the Beta Hunt mine (Figure 2A). At a macroscopic scale, two main components can be identified which are, in order of abundance: quartz and coarse gold. The quartz is crystalline and coarse-grained varying between 2 mm to 1 cm in diameter. The gold and carbonaceous material are both interstitial to the quartz crystals. Consequently, the shape of the gold grains follows the shape of the outer boundary of the quartz crystals. Minor dark telluride-minerals are also locally associated to gold in the sample. Gold is also found in secondary fractures within the quartz.
At a microscopic scale, the interstitial texture of the gold can be easily identified on the backscatter electron images of the sample ( Figure 2B). The backscatter images reveal the presence of inclusions within the gold grains, which vary from less than 1 µm to 3 µm in diameter. Only limited number of the inclusions appear to contain silica phase. Three inclusions were targeted in a gold grain due to their larger size and because their EDS spectra indicated the presence of silica ( Figure 2C).

Deposit details
Discovery is a past producing mine located in the Slave Province in the Northwest Territories of Canada in the northern part of the Yellowknife Greenstone Belt. The Discovery mine has produced ~1 million ounces 13 of mostly high-grade visible gold in quartz veins, hosted in highly silicified and sulfidized metasedimentary rocks which were metamorphosed to the upper greenschist to amphibolite facies 14 . The quartz veins generally trend NW and dip SW 14 . The mineral paragenesis includes gold, arsenopyrite, ilmenite, biotite, pyrrhotite, chalcopyrite, galena, sphalerite and pyrite 14 . A fluid inclusion study conducted on quartz-carbonate veins related to gold at the Giant deposit, which contains similar mineralisation and is hosted within the same Yellowknife greenstone belt, estimated that mineralisation was emplaced at temperatures near 350°C and pressures that ranged from 100 to 200MPa which corresponds to depth of formation greater than 4 km deep 2,15 . The age of mineralisation is estimated at 2591 ± 37 Ma obtained from the similar, nearby Con deposit in the Yellowknife Greenstone Belt using Re-Os geochronology on pyrite associated with mineralisation 16 .

Sample description
The sample is a gold-rich quartz vein ( Figure 3A) with coarse-grained translucent, grey quartz, coarse gold grains interstitial to the quartz grains, biotite, pyrrhotite, galena and carbonaceous material, which are also interstitial to the quartz grains.
At a microscopic scale, the gold is interstitial to the quartz grains and is also found within micro-fractures in quartz. Pyrrhotite, galena and minor chalcopyrite are intergrown with gold and interstitial to the quartz grains ( Figure 3B). Gold contains numerous sub-rounded carbon-rich inclusions (identified by EDX), ~ 1 vol.%, which are generally around 2 µm in diameter but can be as much as 70 µm long ( Figure 3C). Different gold habits are present in this sample; gold occurs in equilibrium with quartz and also within biotite cleavages and in inclusions in pyrrhotite (Figure  3 B,C and D).

Deposit details
Callie is a producing mine that is located in the Granites-Tanami Gold Province in the Northern Territory of Australia; it is world-class deposit with a current total endowment of c. 14.2 M oz 17 . The mineralisation is characterized by high-grade visible gold hosted in quartz veins that can contain up to 10,000 g/t of gold 17 . The high-grade mineralisation is hosted in NE-trending, SE-dipping veins interacting with finely laminated, decarbonized siltstones which were metamorphosed up to amphibolite facies. The mineral paragenesis associated with mineralisation consists of quartz, plagioclase, chlorite, biotite, calcite, traces of sulfides (pyrite, chalcopyrite, arsenopyrite and galena), gold, ilmenite, rutile, monazite, apatite and xenotime 18 . A fluid inclusion study conducted on mineralised quartz veins at the Callie deposit estimated that the veins were emplaced at pressures that range from 70 to 130 Mpa and temperatures that range from 209 to 404°C 19 , which corresponds to a depth of formation greater or equal to 3 km deep. The mineralisation was dated at c. 1805 using U-Pb geochronology on hydrothermal xenotime associated with gold 18 .

Sample description
This sample was previously studied and results are published in Petrella, et al. 20 . This study consists in a re-analyses of the same foil used in Petrella, et al. 20 . The sample is a high-grade (~1,000 ppm Au) gold vein hosted in a decarbonized metasedimentary rock ( Figure 4A). The coarse gold grains are interstitial to quartz and contain microinclusions less than 3 µm, Au NP were identified in one of the inclusion extracted from the Callie gold ( Figure 4B,C).

Deposit details
The Sixteen to One deposit is a past producing mine situated in the Alleghany district within the Sierra Foothills gold province in the central Sierra Nevada Foothills Metamorphic Belt in California, USA. The gold deposits in the Sierra Foothills are structurally controlled and are described as orogenic-type mineralisation 21 . The Alleghany district produced approximately 34.3 tons of very high-grade visible gold 22 hosted in quartz veins 23 . The mineral assemblage at the Sixteen to One deposit consists of free gold in quartz veins associated with pyrrhotite, pyrite, arsenopyrite, sphalerite, chalcopyrite, tetrahedrite, and galena, as well as ankerite, mariposite, and graphite 24,25 . A fluid inclusion study on similar gold deposits in the Allegany district estimated that mineralisation was emplaced at temperatures ranging from ~200 to 300°C and pressure that ranges from ~37 to 250 MPa 26 which corresponds to a depth of formation greater or equal to 1.5 km deep. The mineralisation is mainly hosted in E-dipping veins associated with N-S trending first-order structures 23 which intersect Carboniferous metasedimentary rocks and serpentinized ultramafic dikes 24 . Mineralisation was dated at 114.7 ± 1.4 Ma using 40 Ar/ 39 Ar geochronology on mariposite (Cr-rich muscovite) 25 .

Sample description
The sample is from a gold-rich quartz vein ( Figure 5A) and consists of white, coarse-grained quartz, with coarse gold grains, carbonaceous material, arsenopyrite, galena and feldspars.
Electron microscope investigation shows that gold occurs as free grains interstitial to quartz or associated with arsenopyrite as rims or within micro-fractures ( Figure 5B). Numerous (up to 1 vol.% of the gold) sub-rounded carbon or silica inclusions are present in the gold grains, generally ~ 2µm in size ( Figure 5B).

Investigation by Transmitted Electron Microscopy (TEM)
For TEM investigation, thin samples (foils) were carefully selected and extracted from selected gold grains in each sample. This study is focused solely on coarse gold grains and gold that appears to be late or remobilised was not included. The foil location was determined by the presence of inclusion in gold at the surface of the sample because in our previous study NPs were found preserved in inclusions in gold grains. Table 2 below.

Red lake sample
Free gold grain in equilibrium with quartz was selected for TEM analysis from the Red Lake sample. The gold grains which texture suggests a late deposition or local remobilisation of the gold, such as gold hosted in sulphide microfractures, were not integrated in the TEM study. Further details on the studied inclusion location and composition are included in Figure 6. EDS element maps show the distribution of selected elements in the samples. Please note that individual element maps might locally show artefact in gold (presence of elements within the gold that are not there). There are three problems causing the artefacts: 1) in some areas two phases of different composition overlap; 2) there is a peak overlap between Au, Ag and C in lower energy region, and 3) there is a strong scattering due to Au phase presence.

Beta Hunt sample
The gold grain investigated is interstitial to and in equilibrium with quartz, gold located in fractures cross-cutting the quartz veins were not considered for this study. Three inclusions in one gold grain were extracted for TEM analysis (Figure 2 C). Gold NP were observed in an amorphous silica phase within the upper part of one inclusion (Figure 7). The NPs are sub-rounded to elongated and the largest is ~ 20nm long, other smaller NPs size is below 5 nm ( Figure  7D)

Discovery sample
Two foils were extracted from the Discovery sample in coarse gold grain in equilibrium with quartz (Figure 3 C), gold in inclusion in pyrrhotite and biotite was included in the study.
Foil 1: Further details on foil 1 location and composition is included in Figure 8. The EDS element maps of the inclusion indicates that the amorphous phase is composed of silica and the four NPs encapsulated in the amorphous silica are composed of Au (supplementary information) Foil 2: The composition of each phase in foil 2 is confirmed by EDS element maps and the crystalline state is confirmed by HRTEM and FFT diffractograms (Figure 9).

Callie sample
This study presents a re-analyse of a foil from the Callie sample presented in our previous work 20 . Further details on the foil are included in Figure 10.

Sixteen to One sample
One foil targeting one inclusion was extracted from a coarse gold grain ( Figure 5B) from the Sixteen to One sample and further details on the inclusion composition are included in Figure 11 and Figure 12.    1 showing AuNP in the middle of the image followed by EDS elemental map overlying the HAADF image showing that the inclusion is filled with silica. Please note that the presence of C in the gold is an artefact due to a peak overlap between Au and Ag N peak and C K peak.

Quantitative STEM-EDS measurements
The table below (Table 3) shows the results of compositional analysis obtained on different phases in foils from the samples studied. Cu and Ga have been excluded from the result table because the presence of both elements could be due to contamination induced by the FIB sample preparation process (note that the foil sample holder is made of Cu). The quantitative measurements presented in Table 3 were extracted from specific areas in each foil. The areas selected are indicated by polygons in the EDS multi-element maps for each foil below.

Foil 3
The data extracted from Beta Hunt foil 3 includes coarse gold composition, amorphous silica (aSi) and amorphous carbonic phase (aC). The areas from where the phases composition was extracted is presented in Figure 13. Please note that the amorphous carbonic phase is thin and is overlapping with the coarse gold (Au) directly adjacent to the inclusion.

Foil 2
The data extracted from Beta Hunt foil 2 includes coarse gold composition, amorphous silica (aSi), amorphous carbonic phase (aC) and Fe-bearing phase. The areas from where the phases composition was extracted is presented in Figure 14. Please note that here is an overlap between the amorphous carbonic phase, the amorphous silica (aSi) and the iron-bearing phase that prevents precise measurement of the composition of silica and iron phases.

Foil 1
The data extracted from Discovery foil 1 includes coarse gold composition and amorphous silica (aSi). The areas from where the phases composition was extracted is presented in Figure 15.

Foil 2, area 1
The data extracted from Discovery foil 2, area 1 includes coarse gold composition, amorphous silica and amorphous C. The areas from where the phases composition was extracted is presented in Figure 16.
Please note that the carbon-rich area analysed is overlapping with amorphous silica and coarse gold therefore, the result cannot give a precise composition of the carbon-bearing phase. The data still shows the association of O and N with the amorphous carbonic phase.

Foil 2, area 2
The data extracted from Discovery foil 2, area 2 includes coarse gold composition, Ag2O NP and amorphous carbonic phase and amorphous carbonic phase. The areas from where the phases composition was extracted is presented in Figure 17. The area over the Ag2O nanoparticle contains an overlap with the amorphous carbonic phase therefore, the results cannot precisely indicate the composition of the nanoparticles but indicates a higher concentration in Ag relative to Au.

Red Lake sample
The data extracted from Red Lake foil includes coarse gold composition, Ag2O NP and amorphous carbonic phase. The areas from where the phases composition was extracted is presented in Figure 18.

Callie sample
The data extracted from Callie foil includes coarse gold composition and amorphous carbonic phase. The areas from where the phases composition was extracted is presented in Figure 19.

Sixteen to One sample
The data extracted from Sixteen to One foil includes coarse gold composition and micro-crystalline carbon. The areas from where the phases composition was extracted is presented in Figure 20.  Figure 21 summarizes the composition of amorphous carbonic phase acquired in inclusion from the Beta Hunt sample, the Callie sample, the Discovery sample and the Red Lake sample including the relative proportion of O, C and N content in At. %.